Articulated arm robots have rotary joints that include supporting bearings, and speed reducers connected to prime movers such as electric motors. Position feedback for the joint is usually provided by resolvers or encoders connected either to the motor shaft or to one of two arms that interface at the joint. On some joints brakes are required, usually on the high speed motor side of the reducer, to hold the robot arms from falling under gravity. When such components are utilized as independent components, cost is associated with the use of multiple bearing sets in the joint, motor, encoder, and/or brake. Additional cost is associated mechanical interfaces and couplings between the components; the joint also becomes bulkier and loses its modularity.
Occasionally some components are integrated into fewer modules. For example, servo motors may include an encoder and/or a brake as part of one motor module. This simplifies the assembly and maintenance of the robot and reduces its overall size and cost. Direct drive motors do not need reducers, but are limited to relatively low torque applications. Some other modules integrate the joint bearing with the speed reducer, such as the commercially available Cycloidal and Harmonic drive speed reducers; by adding a motor-brake-encoder module, a robot joint can be built with only two modules.
While speed reducers have been used extensively for robot joints, they have not fully supported the particular needs of robot construction. For example, it is desirable that the robot joint be hollow to pass cables and service lines through the joints from a base to serially connected joints of the robot arm and finally to its end effector. It is also desirable to avoid the duplication of bearings at the joints wherein the robot joint, the speed reducer, and the motor have their own independent bearings.
Robots are also subjected to occasional overloads resulting from interference with peripheral equipment. Such interferences could damage the robot and render it inoperable unless the robot joints and reducers have inherently high overload carrying capacities. Furthermore, the robot operation under tight controls demands a robot joint having minimal backlash and high stiffness. Robot construction also benefits from the integration of components to simplify manufacturing, assembly, and maintenance and reduce robot cost.
Some commercial speed reducers offer some of these advantages. For example, rotary vector reducers, known as RV reducers, and harmonic drives can be integrated into the robot joints and share common bearings. RV reducers also have low backlash and high overload carrying capacity and can be provided with a small center hole. However, the RV reducer is heavy in weight, costly, and does not offer a hollow center core large enough to be of appreciable value. Harmonic Drives have excessive backlash, low overload capacity, and, do not provide a hollow center. Both drives have very limiting motor mounting options. Custom reducers, utilizing gear trains, can be built to provide the required characteristics but at undesirably high costs or with excessive compliance.
U.S. Pat. No. 4,904,148 to Larsson discloses a robotic arm for an industrial robot which includes joints appearing to have a cable passage therethrough.
U.S. Pat. No. 5,069,524 to Watanabe et al discloses a robotic hand and optical fiber connector coupling assembly having passages for the optical fiber cables and various pipes.
U.S. Pat. Nos. 4,918,344, 4,850,457, 4,690,010 and 3,239,699 to Chikamori et al, Taig, Matsumoto et al, and Ferrary, respectively, disclose combined electric motor and speed reducer mechanisms.
Consequently, it is highly desirable to have a single module that integrates all of the components of a rotary joint, such as bearings, motor, speed reducer, brake, and encoder. Such a joint is believed to be disclosed in a pending patent application owned by NASA and having inventors John M. Vranish, Paul W. Richards and Peter D. Spidaliere.
Such module can be produced as a unit at lower cost, have fewer components with high reliability and a compact package. It allows robots to be built more expeditiously, have low maintenance, and be of lower overall cost.
In general, it is desirable that such rotary joints are inherently compact and light weight, have built in encoders and brakes, be capable of accommodating variations in motor mounting locations, and have speed reducers of low backlash that provide optional speed reducing ratios, high overload capability and a large hollow center. An optimum robot joint would integrate in a single module all such desirable joint elements and functions and have a simple mechanical interface at two surfaces for coupling to two adjacent robot arms, or other purely structural parts.